Q: how do i fix it?
A: What kind of mouse is it? is it an optical one? or a mechanical one that uses ball and rollers if it's a mechanical one Dust can cause this effect. What i suggest you do is, Open the roller and clean balls with a soft cotton. Blow/dust free inside the space of the ball. This can work out for you. If it's an optical one, i think it's time for you to get a new one. But at times they can be funny, Plug in the mouse in a different machine and it may work. Make sure you have a good mouse pad.